58 GENERAL ANATOMY OF THE TISSUES. 



interior, are perhaps the reasons why the cells develop no new circum- 

 ferential layers. It is not necessary to carry out this last possibility 

 any further, nor to bring forward the difficulties which are opposed to 

 this view also, among which not the smallest is, that the organic deve- 

 lopment of vesicles does not stop at the formation of nuclei, but is only 

 finished with the completion of the cell membrane ; since in any case 

 the facts brought forward are more than enough to demonstrate the 

 insufficiency of Schwann's hypothesis. I do not see, however, any- 

 thing better or more positive to substitute in its place, and I therefore 

 think it will be most expedient simply to group together the ascertained 

 facts into a few general propositions, which may, perhaps, be done as 

 follows. 



1. The nucleus of the cell arises in the first place, as a precipitate in 

 an organizable fluid, and afterwards becomes consolidated in such a 

 manner, that a special investment and contents with a nucleolus appear. 

 Its development may in this case be compared to that of inorganic pre- 

 cipitates, yet the constantly globular figure, and the size of nuclei 

 which are just formed, indicate some essential though not yet recognized 

 condition peculiar to them. Secondly, cell-nuclei are produced endo- 

 genously in nuclei, or by their division under the influence of a nucleo- 

 lus, which also divides. Here is one condition which is never presented 

 by crystals, the division from an internal cause ; while the other, the 

 influence of the nucleoli upon the nucleus, can hardly be comprehended 

 in any but a physical way, as a molecular attraction proceeding from 

 the nucleoli, of an indefinable nature, which at last draws the entire 

 half of the parent nucleus within its influence. 



2. In the development of cells by division, the cell-nucleus plays 

 exactly the same part which was previously ascribed to the nucleolus, 

 and the occurrence of the formation of cells in this manner demonstrates 

 that chemical conditions are not necessarily concerned therein. 



3. In cell development around portions of contents, and in the 

 cleavage process, the nuclei also operate as simple centres of attraction 

 (einfach attrahirend) upon a certain mass of blastema, and then follows 

 the formation of a membrane upon the surface of this mass, which is 

 most simply understood as a condensation of the blastema. 



4. In cell-development directly around the nucleus, the investment with 

 blastema is wanting, and the nucleus develops the membrane imme- 

 diately around itself. This process admits of both a physical and a chemi- 

 cal explanation. In the first place, we may with Schwann assume 

 that the nucleus attracts molecules, which, when they have reached a 

 certain amount, condense into a membrane, and, by growing, become 

 detached from the nucleus. Or secondly, it is conceivable, that the nucleus 

 in some manner initiates chemical processes, which terminate with the 

 formation of a membrane around it. In this way a coagulable sub- 



